The study of the interstellar medium (ISM) in galaxies has reached a very exciting stage as it has become feasible with present day instruments to observe nearby galaxies at similar resolutions as has been hitherto reserved to studies of the Galaxy. By looking at nearby systems it is now possible to complement our very detailed but necessarily restricted view of the local ISM. The restrictions are mostly caused by our unfavorable position. As we are located within the disk we suffer from line of sight confusion and are hindered in determining accurate distances within the Galaxy. In addition, at optical and shorter wavelengths the line of sight in the disk becomes prohibitively small.
In this review I will mainly deal with the ISM as it is traced by its major constituent, neutral atomic hydrogen or HI. Recently two outstanding reviews have been written by Kulkarni and Heiles (1987, 1988) who describe in detail the diffuse ISM in our Galaxy and the rôle which is played by HI. Another fine review concentrating on the global HI properties of galaxies was written by Giovanelli and Haynes (1988). I will assume that the reader is familiar with these reviews. What I hope to achieve is to describe the structure of the ISM in several of the nearby galaxies which were described by Giovanelli and Haynes, but in as much detail as possible and in the terms used by Kulkarni and Heiles.
In order to limit the scope of this paper I will refrain from describing the kinematics and large scale dynamics of galaxies nor will I discuss at any length spiral structure or spiral density waves. Obviously, as this review is concerned with the atomic gas it will be dealing with the extreme Population I or, in other words, the disk component of spiral galaxies. As a result I will ignore their bulges. For the same reason I will not cover E and S0 galaxies which are really in a class of their own. High sensitivity surveys have recently revealed the presence of detectable amounts of HI in several early type systems. There are many indications that a lot of this gas is due to infall, i.e., it is not necessarily remnant material from which the galaxy was formed (but see also the contribution by G. Knapp in this volume). For a statistical analysis the reader is referred to Knapp et al. (1985) and Wardle and Knapp (1986).
There are, of course, other ways to probe the atomic gas component such as via emission lines in the infrared which can be used to study the higher density neutral gas which occurs near photodissociation regions (see for example Wolfire et al. 1990). A more direct way to determine hydrogen column densities is by observing the Lyman- absorption line in the UV. This technique has been successfully applied to look for neutral hydrogen in the Galaxy (Shull and Van Steenberg 1985), the LMC (Koornneef 1982) and the SMC (Bouchet et al. 1985). The advantage of this technique is that HI column densities can be determined which are independent of saturation effects or spin temperature. The disadvantage is that it only samples HI along lines of sight towards individual objects. Also, one will need powerful UV satellites in order to push this method to the nearest galaxies.
The structure of this review is as follows. It starts with a brief description of the basic radiation transfer equations which are needed to interpret HI observations. After that the main properties of some of the instruments which have opened up this new field of research are discussed. This is followed by a description of the structure of the ISM as we now know it, mainly from observations of the Galaxy. Finally a description is given of the new HI observations of about a dozen nearby galaxies, explaining what they have taught us thus far and indicating which questions one might hope to get answered in the foreseeable future.